This invention relates to an antenna unit and, in particular, to an antenna unit for receiving a radio wave from an artificial satellite (which may be called a “satellite wave” hereinafter) or a radio wave from a ground station (which may be called a “ground wave” hereinafter).
In recent years, a digital radio receiver for receiving a satellite wave or a ground wave so as to listen to digital radio broadcasting has been developed and is put into practical use in United States of America. Specifically, two broadcasting stations called XM and Sirius provide radio programs on 250 or more channels in total. The digital radio receiver is generally mounted on a mobile object such as an automobile and is adapted to receive a radio wave having a frequency of about 2.3 GHz as a reception radio wave to listen to the digital radio broadcasting. In other words, the digital radio receiver is a radio receiver capable of listening to mobile broadcasting. Since the frequency of the reception radio wave is about 2.3 GHz, a reception wavelength (resonance wavelength) λ is about 128.3 mm. It is noted here that the ground wave is a radio wave obtained by receiving the satellite wave at a ground station, slightly shifting the frequency of the satellite wave, and re-transmitting the satellite wave as a linear polarized wave. Thus, the ground wave is the linear polarized wave exhibiting linear polarization while the satellite wave is a circular polarized wave exhibiting circular polarization.
An XM satellite radio antenna apparatus normally serves to receive circular polarized radio waves from two stationary satellites and, in an insensitive zone of the circular polarized radio waves, receives a radio wave by using a ground linear polarization portion of the radio antenna apparatus. On the other hand, a Sirius satellite radio antenna apparatus normally serves to receive circular polarized radio waves from three orbiting satellites (synchronous type) and, in the insensitive zone, receives a radio wave by a ground linear polarization portion of the radio antenna apparatus.
As described above, the radio wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting. Therefore, an antenna for receiving the radio wave must be located outside as known in the art. If the digital radio receiver is mounted to the mobile object such as the automobile, the antenna unit must be attached to a roof of the mobile object (car body).
The antenna unit comprises an antenna and an antenna case for covering the antenna. The antenna case comprises a dome-like top cover and a bottom plate. The antenna comprises an antenna element, a circuit board, and a shield case. For example, the antenna element comprises a patch antenna and receives the satellite wave. The circuit board is provided with a circuit (hereinafter will be called a signal processing circuit) for performing various kinds of signal processing, such as signal amplification, upon a signal received by the antenna element. The shield case serves to shield the signal processing circuit.
In the antenna unit already known, a mechanism (unit fixing member) for fixing the antenna unit to the roof of the mobile object (car body) must be formed by a component inside the antenna case. Furthermore, the antenna must be disposed inside the antenna case so that another component (antenna fixing member) for fixing the antenna to the bottom plate is required also. Thus, the antenna unit already known requires the two components, i.e., the unit fixing member and the antenna fixing member so that a required space inside the antenna case is increased. As a result, the antenna case is adversely affected in designability.